The invention relates to a method of manufacturing a fiber-optical coupling element. The coupling element comprises at least two monomode fibers fused together over a given length. Each fiber comprises a fiber core of core glass and a fiber cladding of cladding glass. In the fibers, the cladding glass has a refractive index and a softening temperature, both of which are lower than the refractive index and the softening temperature of the core glass.
The two fibers are positioned so that their parts to be fused are in contact with each other and are pressed against each other. The fibers are then heated at the parts to be fused in a manner such that the cladding glass softens, the spacing between the fiber cores is reduced, and the two fibers are fused together without softening the fiber cores. The fibers thus fused together are fixed to form a coupling element by cooling to room temperature.
Coupling elements thus manufactured are used in fiber-optical data transmission systems with one or more loops which are provided at different areas with such coupling elements to add or to split off a signal.
Normally, the fiber cladding of optical fibers is made of a cladding glass having a higher softening temperature than that of the core glass. This is because in the combinations of glasses most frequently used for the fiber cladding and the fiber core, the lower refractive index of the cladding glass goes hand in hand with a higher softening temperature, and the higher refractive index of the core glass corresponds to a lower softening temperature.
When two of these standard fibers are now fused together along their peripheries, the fiber cores are softened and deformed. This results in an uncontrolled escape of light from one fiber core, which light is not collected by the other fiber core. The expression deformed is to be understood herein to mean an undesirable deformation of the original cross-section of the fiber core or an uncontrolled change of the form of the fiber core which is causes excessively large optical losses.
Deformation of the fiber core is avoided in a method known from U.S. Pat. No. 3,902,786 (Brown). According to Brown, the claddings of the fibers are removed at their parts to be coupled. The fibers are then positioned so that their bare cores contact each other. The bare cores are then coated with a material whose refractive index is equal to that of the cladding material.
The method of Brown is rather objectionable because bare fiber cores, especially of monomode fibers, the cores of which each have a diameter on the order of 5 to 9 .mu.m, cannot be handled practically.
Deformation of the fiber cores is also avoided in another method known from U.S. Pat. No. 4,054,366 (Barnoski et al). Deformation is avoided by heating the fiber claddings locally, at the area at which the fibers are to be fused, by means of a laser beam. This method is particularly intended for fusing together fibers having cores whose diameters are on the order of 30 to 100 .mu.m and whose softening temperatures are lower than that of the cladding glasses.
In the method described in the first paragraph and known from British Patent Application No. 2,030,318, softening of the fiber cores is prevented by the use of fibers whose cladding glass has not only a lower refractive index but also a lower softening temperature than the core glass. However, no further data about the softening temperature and the viscosity of core glass and cladding glass are stated in this patent application.